1. Field of the Invention
This invention relates to a line light source system which can be used, for instance, a light source system for irradiating a line-like area of an image recording medium.
2. Description of the Related Art
There have been known systems in which a photoconductive member such as a selenium plate (comprising an a-Se layer sensitive to X-rays) is employed as an electrostatic recording medium (sometimes referred to as “a solid sensor”) in order to reduce the amount of the radiation to which the examinee is exposed, the electrostatic recording medium is exposed to radiation such as X-rays carrying thereon a radiation image, thereby causing the electrostatic recording medium to store latent image charges representing the radiation image, and then the latent image or radiation image information representing the radiation image represented by the latent image charges is read by causing a laser beam to scan the electrostatic recording medium and detecting the electric current generated in the electrostatic recording medium upon exposure to the laser beam by flat electrodes or stripe electrodes. See, for instance, U.S. Pat. Nos. 4,176,275, 5,440,146 and 5,510,626 and “A Method of Electronic Readout of Electrophotographic and Electroradiographic Image”; Journal of Applied Photographic Engineering Volume 4, Number 4, Fall 1978, pp. 178 to 182.
We, this applicant, have proposed an electrostatic recording medium comprising a first conductive layer permeable to recording radiation, a recording photoconductive layer which exhibits conductivity upon exposure to the recording radiation, a charge transfer layer which behaves like a substantially insulating material to a charge in the same polarity as that in which the first conductive layer is charged and behaves like a substantially conductive material to a charge in the polarity opposite to that in which the first conductive layer is charged, a reading photoconductive layer which exhibits conductivity upon exposure to reading light and a second electrode layer permeable to the reading light which are superposed one on another in this order is used, and a read-out apparatus for reading radiation image information from the electrostatic recording medium on which the radiation image information has been recorded. See, for instance, Japanese Unexamined Patent Publication No. 2000-105297.
In the read-out apparatus disclosed in Japanese Unexamined Patent Publication No. 2000-105297, an electrostatic latent image recorded on the electrostatic image recording medium is read by causing reading light to two-dimensionally scan the electrostatic recording medium. The light source systems outputting the reading light include, for instance, a spot beam projecting means which causes a spot beam, e.g., a laser beam, to scan the electrostatic recording medium in both the main scanning and sub-scanning directions, and a line beam projecting means which causes a line beam extending in the main scanning direction to scan the electrostatic recording medium only in the sub-scanning direction. The line beam may be emitted from a line light source comprising, for instance, a number of linearly arranged light emitting elements.
As the line light source system, there has been known that in which a number of LED's are arranged in an array as disclosed, for instance, in Japanese Unexamined Patent Publication No. 2001-290228. LED's are high in light output efficiency for given input energy and are low in cost as compared with lasers or the like. When the line light source system of an LED array is employed, light emitted from the LED's is linearly focused on the electrostatic recording medium to project a line beam thereonto by, for instance, a cylindrical lens disposed in parallel to the row of the LED's (the direction in which the LED's are arranged), and the line beam is caused to scan the electrostatic recording medium in the sub-scanning direction.
An example 200 of the reading light source system comprising a line beam projecting means will be described with reference to FIGS. 8A and 8B. FIG. 8A is a side view showing the reading light source system 200 of this example as seen in Y-direction and FIG. 8B is a cross-sectional view taken along an X-Y plane. As shown in FIGS. 8A and 8B, the reading light source system 200 comprises a line light source 101 comprising a plurality of linearly arranged surface-emitting LED chips 101a, 101b, 101c . . . , a slit plate 102 having a slit 102a extending in the longitudinal direction of the line light source 101 and a pair of cylindrical lenses 104 and 105 (an optical means) which image reading light L passing through the slit 102a on an electrostatic recording medium 10 disposed on a glass base 6. The slit plate 102 limits the angle of divergence in the Y-direction of the reading light L as output from the line light source 101. That is, the reading light emitted from the LED chips 101a, 101b, 101c . . . is shaped by the slit plate 102 and converged in the Y-direction by the cylindrical lenses 104 and 105, and then projected onto the electrostatic recording medium 10. Since the reading light emitted from each of the LED chips 101a, 101b, 101c . . . is not converged in Z-direction in which the LED chips 101a, 101b, 101c . . . are arranged (the longitudinal direction of the line beam projecting means), the reading light L irradiates a line-like area of the electrostatic recording medium 10.
However, the conventional reading light source is disadvantageous in that since the angle of divergence of the reading light L is not limited in the direction of the row of the light emitting elements (the longitudinal direction of the line beam projecting means), the line reading light beam L includes therein a plurality of light bundles which are focused on different points when converged in the direction perpendicular to the longitudinal direction of the line beam projecting means by the optical means. Accordingly, in the line beam on the image recording medium, light bundles in focus and light bundles out of focus mingle with each other. Light bundles out of focus increase flares and enlarge the width of the line light beam.
Why the line reading light beam L includes therein a plurality of light bundles which are focused on different points when converged in the direction perpendicular to the row of the light emitting elements by the optical means will be described with reference to FIGS. 9A and 9B, hereinbelow.
FIG. 9B is a plan view showing an example of the line light source system and FIG. 9A is a cross-sectional view taken along line A—A in FIG. 9B. The structure of this example is substantially the same as that shown in FIGS. 8A and 8B. Accordingly, the elements analogous to those shown in FIGS. 8A and 8B are given the same reference numerals and will not be described here.
As shown by broken lines T1, T2 . . . in FIG. 9B, the light bundles L1, L2, L3 . . . comes to be focused by the cylindrical lenses 104 and 105 on a point nearer to the cylindrical lenses in a plane parallel to the row of the LED chips as the distance from the axis of light emission of each of the LED chips 101a, 101b, 101c . . . increases.
Further, there has been a problem that the line reading light beam projected onto the image recording medium is not uniform in its intensity in the longitudinal direction of the line beam projecting means. That is, in a middle portion of the line reading light beam projected on the image recording medium, the intensity of the line light beam is higher since light bundles emitted from a larger number of light emitting elements are superimposed in the middle portion. Since light bundles emitted from a smaller number of light emitting elements are superimposed towards the ends of the line reading light beam, the intensity of the line light beam becomes lower towards the ends of the line reading light beam.
Further, a part of the reading light emitted from light emitting elements disposed near the ends of the line light source is reflected by the casing of the line light source, which increases flares and deteriorates the sharpness of the image information read out.